Within the field of designing planing watercraft, it is generally known to take advantage of the energy of spray water, which is displaced laterally when the hull is moved through sea water, by means of longitudinally extending water-deflecting surfaces, also known as “foils” or “lifting strakes”, which are impinged by the displaced spray water. Numerous patents disclose various kinds of such foils or lifting strakes, e.g. U.S. Pat. Nos. 4,584,959 and 6,666,160 B1. Apart from using lifting foils or strakes, these prior art hulls are further provided with means for air lubrication of the bottom of the hull.
EP 1 685 020 B1 discloses a planing hull that includes a bottom portion and a side portion delimited by edges in the hull known as chines as well as a forward portion and an aft portion. The forward portion and the aft portion are offset relative to each other at a V-shaped step, the point of which is oriented forwardly. The step extends transversely between the two chines. Two tapered lateral skids, each projecting from the bottom of the hull in the vicinity of a chine, and the longitudinal axis of which is substantially parallel to the corresponding chine, are arranged on the aft portion of the hull. The aim of this design is to keep most of the bottom surface aft of the V-shaped step dry at planing speed. Thus, there are no surfaces that will be impinged by laterally moving water.
SE 405 584 (claiming priority from U.S. Pat. Nos. 167,737 and 237,852) discloses a watercraft vessel with a planing hull exhibiting a delta-shaped planing surface, which is exactly flat (0 degree deadrise) and forms a submerged part of the hull. The concave side portions of the hull can, according to the patent, be provided with water deflectors, adapted to re-direct upward flowing spray and/or splash water along the sides of the hull into a downward direction.
Furthermore, in the relevant prior art, there are also basic theories on flow behavior when a body moves through a fluid. Such theories have been presented by Herbert Wagner in “Jahrbuch der Schiffbautechnik”, vol. 34, 1933, “Über das Gleiten von Wasserfahrzeugen” (English translation in National Advisory Committee For Aeronautics, Technical Memorandum, No. 1139, Washington, April 1948), and by Daniel Savitsky et al. in a paper entitled “On The Main Spray Generated By Planing Surfaces” published January 1958 by the Institute of the Aeronautical Sciences, and likewise by Daniel Savitsky et al., in a paper entitled “Inclusion Of Whisker Spray Drag In Performance Prediction Method For High-Speed Planing Hulls” published in Marine Technology, Vol. 44, No. 1, January 2007, pp. 35-56.
The theories presented in these papers form the basis on which the bottoms of modern high-speed watercraft are designed. The latter paper includes the following important information for a boat designer (col. 2, 1st paragraph):
“The present paper identifies the whisker spray (its area, flow direction and location); quantifies its contribution to total drag as a function of trim angle, deadrise angle and speed; and incorporates these results into a final prediction procedure. Further, the designer is given guidance as to the location, size, and geometry of spray strips that will deflect the whisker spray away from the bottom.”
This knowledge forms the basis for most prior art designs of “spray strips” which deflect water to the sides (and in some cases slightly downwards) in order to create lift and reduce the wetted surface that causes friction.
When a planing hull moves through the surface of sea water, the water immediately adjacent the hull cannot be displaced laterally due to the inertia and the incompressible nature of the medium. This phenomenon is usually referred to as “wave rise” and is also the root cause of spray on the sides of a high-speed watercraft vessel. The formation of spray has been recognized as a problem since it is an essential part of the resistance of a planing boat (“The Spray Volume Shed by an Uncambered Planing Hull in Steady Planing”; Payne 1982). However, this problem has not been solved and seems to be ignored in all prior art planing hull designs. In any case, there are no known analyses of the potential to make positive use of the energy contained in the lateral spray.
According to Wagner (Jahrbuch der Schiffbautechnik), the wave rise increases the wetted region to create a stagnation line, where the flow speed is the lowest and the water pressure is the highest, above the hull/water surface intersection. This increased wetted region makes up the root for whisker spray.
The stagnation line is located inside this increased wetted region. The spray root, if seen from underneath, can normally be seen approximately as an isosceles triangle without a base with its tip pointing towards the bow. Its angles are not directly dependent on speed, but on the trim angle and the deadrise angle.